Acetylation of wood pulp



Patented Jan. 18, was

PATENT ()FFICE AGETYLATION F WOOD PULP Arthur N. Parrett and ton, Wasln,

Judson H. Holloway, Shelasslgnors to Rainier Pulp & Paper Company, San Francisco, Calif., a corporation of Delaware No Drawing; Application June 27, 1936, V serial No. 87,761

Our invention relates to the art of acetylation of wood pulp for the manufacture of cellulose acetate. Particularly does our invention apply to the acetylation of high-alpha-cellulose wood pulp. ,By high-alpha-cellulose wood pulp is usually'meant such wood pulp having not less than about 94% alpha cellulose. 'While our invention, as presently appears, will have its greatest application in the preparation of high-alpha cellulose for acetylation, it is to be understood as applicable to other refined wood pulp.

At the present time cotton linters are used universally in the manufacture of cellulose ace tate. On account of the high cellulose purity of cotton linters, manufacturers are able to acetylate cotton linters directly. In other words, commercial cotton linters are quite active towards acetylation, or at least do not resist acetylatlon. However, in the case of wood pulp, such as high alpha pulp, there is a marked tendency towards inactivity or resistance to acetylation. In the early days of the acetylation industry considerable difiiculty was encountered due to the inactivity of cotton linters but improved methods of manufacturing cotton linters have eliminated this difficulty. The development of methods which eliminate this difilculty for alpha wood pulp has not occurred and it is highly important to devise a method by which such wood pulp may be acetylated satisfactorilyand economica 13.

The manufacture of wood pulp occurs generally in those sections ofthe country heavily forested. Wood pulp,

established practice, and as suchis'subjected to drying athigh temperatures to eliminate all the moisture practically possible for various reasons-for example, to properly keep the product and prevent molding and spontaneous decomposition, and also to de-, crease its weight and bulk to the minimum for economical and convenient shipping. Accordingly, the practice has long been to ship such wood pulp in sheet form, in rolls,'or in bundles of according to long and well fiat sheetsto manufacturing centers for conversion into paper, nitrocellulose and other products. Suchwood pulp in said sheet form, having a moisture content of about 6% to 10%, is characterized by being inactive towards the acetylating mixture which is composed'of acetic acid, acetic anhydride and sulphuric acid.

A primary object of our invention is to provide wood cellulose in such comminuted form as is suitable for acetylation and which at the same time will be reactive to thereagents constituting Billaims. (c1. 260-101) is prepared in sheet form the acetylating mixture. Another object of our invention is to provide a process which will apply to sheets of the wood pulp and render the same reactive to the acetylating mixture and this in the comminuted form. That is, an object of our 5 invention is to retain the highly advantageous and economical method of shipping wood pulp in the form of sheets because of compactness, convenience of handling, etc., and to not discard, but rather conform with, as closely as may be desired, the established method of producing wood pulp and drying it in sheet form. Also, it'is an object of our invention to provide such a process for preparing wood pulp for acetylation as will be characterized by its emciency, thoroughness of 15 treatment and, at the same time, economy.

As explained, the sheet of high-alpha wood pulp as presently manufactured is not employed for making cellulose acetate, cotton linters universally constituting the raw material for the manufacture of such acetate. Such wood pulp in said sheet form is not adapted for such use because it is inactive to the acetylating mixture.

We'have found,'ai'ter extended experimentation, that the temperature at which the cellulose 35 is dried is one of the critical factors. It must be dried at a temperature below 35 C. if wood pulp is to be rendered active towards the acetylating mixture. Wood pulp dried in sheet form according to established methods of manufacture is subjected on the paper machine. to relatively high temperature, such as C. to C. We disthe acetylating mixture. But since such wood pulp is in the sheet form, it is not suitable for acetylation in manufacturing cellulose acetate. In order to acetylate wood pulp, it must, first of all, be in a dissemmated or disintegrated form 40 so that it may be (a') mixed with the, acetylating agents, (b) brought into the most intimate relation therewith, that is, each individual fiber must be freely exposed to the acetylating mixture, and

(c) the fibers must be thoroughly agitated while 45 exposed to the said acetylating agents.

In subjecting the above described wood pulp sheet which had been carefullydried at said low temperature, to mechanical disintegration, an unexpected dlfllculty developed. We found that 50 even though the cellulose is dried while being formed in-sheets at low temperature, such as less than 35 (3., as contrasted to about 100 C. as commonly employed in drying the sheets. and then subjected to disintegration by any one of a 55 sheared with a knife.

number of well known mechanical devices, such as attrition mills, hammer mills, shredders etc., that such comminuted wood pulp is inactive to theacetylating mixture. In other words, the very process or act of comminuting renders wood pulp, that is reactive to the acetylating mixture, inactive. The fact that mechanical disintegration of the sheet by any one of a number of different methods creates inactivity towards acetylation, even though the cellulose was carefully dried at low temperature prior to such disintegratiomwas one discovery or result of our experimentation.

Having encountered this obstacle to the practical use of wood pulp for cellulose acetate manu facture, we undertook experimental work along many lines in an effort to search out the cause of this inactivity as well as the means or method of overcoming or avoiding it.

,Types of inactivity: According to generally accepted theory there are several causes of inactivity towards acetylation'. One type may be termed physical inactivity and this may be explained as inactivity due to dense masses of fibers, such that the acetylating agents will not fully or properly penetrate such dense bodies and react chemically with the cellulose. Inactive bodies of fibers may even result from the pinching effect developed when the sheetsare This would lead to incomplete acetylation due purely to failure of the acetylating agents to reach the cellulose in sufficient quantity to bring about complete reaction due to exclusion of the reagents.

Another type of inactivity is ordinarily termed physical chemical inactivity. This type of inactivity manifests itself regardless of the density of the mass of cellulose or the particular structure of the fibers in relation to each other. The most accepted theory explaining this type of inactivity relates to the presence of hydrocellulose as a component of the cellulose which constitutes the individual fibers. Thus a cellulose which is completely or almost completely free from the indefinite type of. cellulose which is termed hydrocellulose shows little or very little "physical chemica inactivity. In the case of highly refined wood pulps or cellulose derived from lignocellulose, there is apt to be more hydrocellulose' present after purification than in the case in cotton. Consequently highly refined wood pulp is more apt to show the phenomenon of physical chemical inactivity.

Our invention eliminates inactivity of all types and it seems justified to explain this elimination of inactivity by reasoning that the physical chemical inactivity is avoided by drying the comminuted pulp at low temperature and the physical inactivity is avoided by comminuting or disintegrating the. pulp at the minimum moisture content which will avoid the development of physical chemical" inactivity because of effects developed in the fiber during the mechanical disintegration itself and avoids physical inactivity by the nature of the mechanical disintegrating apparatus selected for the purpose. In other words, it is only necessary to select a type of disintegrating apparatus which will provide a means of avoiding masses of. cellulose or pulp which will not acetylate because of the theory previously advanced. After extended experimenting, we discovered that if the wood pulp sheet as shipped was propulp is 70% is sufficiently thorough ing agents.-

obtain intimate contact, the

nuted while so moistened, and then dried at a temperature of 35 C. or less, that the cellulose would be reactive to the acetylating reagents to a degree comparable to cotton linters, and would provide a cellulose acetate product having other characteristics which would equal or approximate those of cotton linters. When the moisture content was at a point in the range of 6 to 10%, when mechanically comminuted, we found that the pulp was inactive even though. dried at a temperature below 35 C.

' In practicing our invention, the wood pulp sheets as shipped or manufactured, are first wetted with water or water vapor, so that its moisture content lies between 12% to 30% or the to bone'dry. Such sheet seems to be slightly damp and yet the presence of this amount of moisture provides characteristics for the pulp so that when the sheet of originally inactive wood pulp is comminuted and then dried at a low temperature, i. e., 35 C. or below, that the cellulose is active to acetylating agents.

In comminuting or mechanically disintegrating, we prefer to use that known type of an attrition mill consisting of two iron or steel discs independently rotatively mounted face to face with a slight clearance between them with the pulp fed axially of the discs, so that the pulp is moved centrifugally radially of the discs. This type of mill has at least one of the disc faces corrugated, preferably both. The corrugation may be radially or otherwise disposed-so as to produce impact on the fibrous material when the discs are given rapid directionally differential rotation. However, other types of shredders or hammer mills or cutters work substantially equally well. It is only necessary to provide a method of disintegrating the pulp sheets which so that no particles, pellets or clumps remain which are so dense that they will not be properly wettedby the acetylat- The moisture content may be adjusted from 12% to 30%, depending on the particular type of mechanical disintegration to be used. After disintegrating the pulp into a physical form suitable for acetylation so that it may be mixed with the acetylating agents so as to pulp will still contain an undesirable amount of moisture. Unless this moisture is lowered, an undue amount of acetic anhydride will be decomposed and used up. Hence, we remove as much of this moisture as ,practical by drying the disintegrated pulp in well known types of dryers where the pulp is dried with a blast of air maintained at the desired temperature and humidity. In our case, we maintain thetemperature at 35 C. or below, and dehumidify the air to a low relative humidity.

To summarize, our invention in general comprises the.following steps:

1. Manufacture of high-alpha-cellulose wooddetermined moisture content in the range of 12% and 30%.

4. Mechanically disintegrating the partially wet pulp by one of several different methods.

-5. Drying the mechanically disintegrated pulp at a temperature of 35 C. or below with air of adjusted humidity to a moisture content of the lowest practical figure.

Obviously, Step 3 may be eliminated merely by conducting Step 2 so-t'hat the moisture content of the pulp is controlled as it is produced on the drying machine. This would merely involve operation of the drying machine so that the moisture content of the pulp would end up in the range 12% to or at any predetermined point in this range.

The following experiments andt'he results will prepared for acetylation by the following difierent methods:

Sample A.The dry pulp containing about 6% moisture was fiufied by a rotating picker roll so that the mass of flufi? was free from dense clumps of fibers.

Sample B.-The sheets of pulp were moistened with water, the amount of water added was such that the moisture content was increased from about 6% to about 25%. These sheets containing about 25% moisture were exposed to the atmosphere and allowed to dry for 24 hours at the end of which time the moisture content was found to be about 10%. These air-dried sheets were then fiufied similarly to Sample A.

I Sample C'.The sheets of pulp were moistened with water, the amount of water added was such that the moisture content was increased from about 6% to about 25%. These sheets containing about 25% moisture were then fiuifed by the same means as Samples A and B. The fiuffed' 1 pulp thus produced appeared damp and was alcotton linters, the solutions of cellulose acetatelowed to room temperatureuntil themoisture content fell to about 10%. Y

Samples A, B and C ,were then acetylated with acetic acid, acetic anhydride and'sulfuric acid.

acetylating mixture. No pretreatment step was used as treating with acetic acid. The three acetylations were carried on in glass bottles. When the acetylation was judged to be complete according to conditions ordinarily successful for appeared as follows:

Sample N 0. Appearance oi triacetate solution Sample A An ope ue, grainy mass of unreactod ilbors and partia ly reacted fibers. Sample )3 An opaque, grainy mass oi unreacted fibers and partially reacted fibers, very slightly better than Sample A. Sample 0 A smooth uniform solution free from unrcected 1 fibers and gralnlness. The solution was very slightly milky. Thismilklness or haze was oliviously not due to ieilure to acetylate propor y.

A. number of similar series of experiments have u made in which various types of mechanical disintegration of pulp in sheet form were substituted for the picker rolls used above. Among these were an impact-attrition him, a hammer mill, and a machine designed to cut sheets of pulp into ,55" squares or other small pieces of cleanly cut uniform size. Aha-combinations of the above mentioned machines were used. In every case, the procedure and sequence of steps used in Sample C gave uniform solutions of cellulose triacetate free from undissolved fibers and graininess, whereas procedures used in Samples A and B were very unsatisfactory due to unreacted fibers and graininess.

dry by exposure to the atmosphere at e 3 We claim:

1. The process of acetylating refined wood pulp, comprising providing high alpha cellulose pulp in the sheet form with "a predetermined moisture content in the range of 12 to 30%; substantially exposing the individual pulp fibers by mechanically comminuting said sheet with said predetermined moisture content to permit wetting of the fibers by acetylating agents; drying such comminuted pulp to a relatively low moisture content with air at a temperature below 0.; and subjecting such dried pulp to acetylating agents.

2. The process of acetylating refined wood pulp, comprising manufacturing high alpha cel lulose pulp in sheet form; drying the same at relatively high temperatures to a moisture content of 6 to 10%; providing such pulp with a predetermined moisture content in the range of 12 to 30%; substantially exposing the individual pulp fibers by mechanically comminuting said.

pulp with'said predetermined moisture content to permit wetting of the fibers by acetylating agents; drying such co'mminuted pulp to a relatively low moisture content with air at a temperature below 35 (3.; and subjecting such dried pulp to acetylating agents.

3. The process of acetylating refined wood pulp, comprising forming high alpha cellulose pulp into a sheet having a moisture content greater'than 30%; drying such sheet to a partially wet sheet having a moisture content in the range of 12 to 30%; substantially exposing the individual pulp fibers by mechanically comminuting said partially wet sheet to permit wetting of the fibers by acetylating agents; drying such comminuted pulp to a relatively low mols- I ture content with air at a temperature below 35 C. and subjecting such dried pulp to acetylating agents.

4-. The process oi acetylating refined wood pulp, comprising manufacturing high alpha cellulose pulp in'sheet form; drying the same at relatively high temperatures to a moisture content of about 6%; providing such pulp with a moisture content of about 25%; substantially exposing the individual pulp fibers by mechanically comminuting said pulp having said 25% moisture content to permit wetting oi the fibers by acetylating agents; drying said comminuted pulp to a relatively low moisture content with air at a temperature below 35 0.; and subjecting such dried pulp to acetylating agents.

5. The process of acetylating refined wood pulp, comprising'providing refined wood pulp in 1 the sheet form with a predetermined moisture content in the range of 12 to 30%; substantially exposing the individual pulp fibers by mechanically comminuting said sheet with said predetermined moisture content to permit wetting of the fibers by acetylating agents; drying such comminuted pulp to a relatively tent with air at a temperature below 35 (2.; and subjecting such dried pulp to acetylating agents.

6. The process of acetylating refined wood pulp, comprising manufacturing refined wood pulp in sheet form; drying the same at relatively high temperatures to a moisture content of 6 to 10%; providing such pulp with a predetermined moisture content in the range of 12 to 30%; substantially exposing the individual pulp fibers by mechanically comminuting said pulp with said predetermined moisture content to permit wetting of the fibers by acetylating agents; drying such comminutecl pulp to a relatively low moisture content with air at a temperature below 35 4- I memos (2.; and subjecting such dried pulp to acetylating agents.

7. The process or acetylating refined wood. pulp, comprising forming refined wood pulp into a sheet having a moisture content greater than 30%; drying such sheet to a partially wet sheet having a moisture content in the range of 12 to 30%; substantially exposing the individual pulp fibers by mechanically comminuting said partially wet sheet to permit wetting of the fibers by acetylating agents; drying such comminuted pulp to a relatively low moisture content with air at a temperature below 35 C.; and subjecting such dried pulp to acetylating agents.

8. The process of acetylating refined wood pulp, comprising manufacturing refined wood pulp in sheet form; drying the same at relatively high temperatures to a moisture content of about 6%; providing such pulp with a moisture content of about 25%; substantially exposing the individual pulp fibers by mechanically comminuting said pulp having said 25% moisture content to permit wetting of the fibers by acetylating agents; drying said comminuted pulp to a relatively low moisture content with airat a temperature below 35 C.; and subjecting such dried pulp to acetylating agents.

ARTHUR N. PARREII'. JUDSON H. HOLLOWAYJ 

